Personal alarm systems as presently developed generally fall within two categories. One category is designed to operate in the audio range and provide a loud noise to attract the attention of people in the vicinity who might provide aid and, in some instances, to surprise an attacker. The other type of personal alarm system is the silent alarm which provides no audible signal in the vicinity of the alarm transmitter. Systems of this type generally employ radio signals which are picked up by receivers installed in a protected area, and these systems transmit some type of identification code unique to the transmitter.
Both types of known personal alarm systems are subject to a number of disadvantages. The strictly audio alarm transmitter is in effect useless if there are no people within the range of the audio signal produced. Additionally, strictly audio alarm units are often ineffective in areas of high noise level, and the audio signal may be masked by other audio signals of the same frequency and amplitude. On the other hand, totally silent alarms employing radio frequencies or frequencies beyond the audio range have no deterrent effect upon an attacker and fail to attract the attention of persons within the range of the audio signal who may provide a source of immediate aid. Additionally, radio signals can often be jammed, or spurious radio frequencies in an area can cause a receiver system to provide false alarms.
In any high risk situation, such as those existent in prisons, mental institutions, and public areas such as parking lots and college campuses, as well as in institutions and areas where persons require emergency care, a great need exists for a personal alarm system which is operable in the audio range and is effective to summon aid from the immediate vicinity of the alarm transmitter as well as providing the capability of summoning aid through the means of a central receiver system. Any alarm system operable in the audio range must be designed to effectively eliminate the probability of false alarms being generated by other audio signals in the vicinity, and must also be designed to prevent unauthorized use of the signal generator to defeat the alarm system by triggering repeated false alarms.